The present invention relates to a method of intensifying washing of cellulose pulp in various washing apparatus. The method according to the invention is particularly applicable for use with a DRUM DISPLACER.RTM. washer sold by A. AHLSTROM CORPORATION of Helsinki, Finland, but also in connection with conventional washing presses. The method is also applicable for use with other washing apparatus, and various suitable washers used for the washing of pulp are described herein.
Several different types of washing apparatus and methods are known from the prior art. Arrangements that can be clearly distinguished from each other include diffusers, drum washers and Fourdrinier wire washers. The pulp is fed into diffuser washers at a consistency of about 10%. The feeding consistency of drum and Fourdrinier wire washers is 1-3%. Drum washers in use at present include a suction washer, a wash press, and a pressurized or superatmospheric washer.
A traditional suction washer comprises a drum covered with a wire, which drum rotates in a basin. On the shell of the drum below a perforated plate are collecting compartments, each of which is connected by a tube of its own to a valve system on the shaft at the end of the drum. From the valve the filtrate is led, for example, into a filtrate tank through a barometric leg or a centrifugal pump generating the suction required. The valve arrangement ensures that the suction effect of the barometric leg can be directed to desired points of the web formation section.
Web formation in a suction washer takes place in such a way that inside the drum rotating in the basin, reduced pressure is generated by the barometric leg or another apparatus generating suction, which under pressure sucks pulp suspension against the drum. As the liquid permeates the drum, the fibers of the cellulose pulp precipitate upon the surface of the drum. The consistency of the fiber suspension in the basin is about 0.5-2%, whereas the consistency of the layer precipitated on the drum is about 10-12%. The web formation area, i.e. that part of the rim of the drum which is covered by the fiber suspension in the basin, is about 140 degrees. The maximum rotational speed of such a drum is 2-2.5 revolutions per minute, since at higher speeds the collecting compartments of the filtrate and the tubes will not have enough time to empty.
Washing takes place as a displacement wash in such a way that wash liquid is injected onto the surface of the drum protruding from the pulp basin, which wash liquid soaks through the pulp layer and displaces the majority of the liquid originally present in the pulp. The extent of the displacement area is about 120 degrees. A typical specific square load is about 5-7 BDMT/m.sup.2 /d, so that the thickness of the pulp web is on the order of 25 mm. The square load of the suction washer when used in bleaching is about 8 BDMT/m.sup.2 /d, and the thickness of the web is about 30 mm.
A wash press comprises a drum covered with a wire or having a drilled perforated plate shell. The feed of the pulp takes place at a consistency of 3-4%, and the knots and the like have to be removed from the pulp before the washer. On the shell of the drum are compartments from which the filtrate is fed through a chamber on the end rim. The drum may alternatively or additionally be open, so that the filtrate is collected inside the drum and discharged from an end opening. The length of web formation is about 90 degrees around the drum circumference and that of displacement is about 150 degrees. The speed revolution of the drum is about 2 revolutions/minute, the specific square load being about 15-20 BDMT/m.sup.2 /d. The consistency of the washed pulp may rise up to about 35%. Displacement takes place with the pulp is at a consistency of 10-15%, with the pulp web about 30-50 mm thick.
One example of a superatmospheric washer is that according to FI patent publications 71961 or 74752. This washer comprises a rotating drum and a solid shell encircling it. The drum comprises or consists of a stationary shell housing on its outer surface ribs attached at intervals of about 200 mm. Between the ribs are perforated plates fixed in such a way that the ribs extend outwardly from the perforated plate about 40-60 mm. The ribs, together with the perforated plates, define pulp compartments. Inside the perforated plates and below the pulp compartments, i.e. in the volume defined by the plates, ribs and solid shell, filtrate compartments are formed into which the filtrate displaced by the wash liquid is gathered. At the end of the cylinder drum, substantially on the diameter of the rim of the drum, is a valve system, through which the filtrate is removed and redirected. There are several stages, usually 2-5, provided in the washer. This means that the wash liquid is used several times to wash the pulp. In other words, filtrates gathered in the filtrate compartments are fed upstream from one washing stage to another. Outside the drum of the washer, constituting a part of the shell of the washer, are feed chambers for the wash liquid, from which the wash liquid is pressed through flow controllers into the pulp in the pulp compartments so as to displace liquid from the pulp to effect washing.
Web formation and washing of the pulp are carried out in such a way that the pulp to be washed is fed through a special feed box into the pulp compartments. The feed box may be constructed so that it dewaters the pulp, so that axial "bars" of the same length as the drum are formed in the pulp compartments. Immediately after the feed point there is a first washing zone. Altogether there are four or five separate stages in the washers of the aforementioned patent publications. A wash liquid flow is fed to each stage and it displaces the liquid present in the pulp layer in the compartments of the wash drum. It was already mentioned above that the filtrates are led upstream from one stage to another. In other words, (cf. FI patent 74752, FIG. 1), a clean wash liquid is pumped to the last washing stage and the filtrate displaced by this liquid is led to the second last washing stage to serve as wash liquid. After the last washing stage, the "pulp bars" are detached from the drum, for example by blowing with pressurized air, and transported away via a transport screw.
A specific square load of this kind of superatmospheric washer with four stages may rise up to about 30 BDMT/m.sup.2 /d. If the feed consistency rises, then the specific square load may even rise above 30 BDMT/m.sup.2 /d. The thickness of the "pulp bar" is about 50 mm and the consistency may even rise to 15-18%. The consistency of the pulp fed onto the drum may vary between 3.0-10%. The angular speed of the drum varies between 0.5-3.0 rpm.
U.S. Pat. Nos. 4,919,158 and 5,116,423 disclose an arrangement which is somewhat more advanced than the basic approach of FI patent 71961, and which can achieve a remarkably improved wash compared to the basic arrangement of the above-mentioned publications. In the arrangement of FI patent 74752, each washing stage is divided into two zones in such a way that two wash filtrates with different concentrations are obtained from each stage. Filtrates so obtained are fed upstream in the manner disclosed in these patents. Further, the patents describe how a suction filtrate, i.e. the filtrate extracted from the point between the last washing stage and the pulp discharge, is removed with the washing filtrate from the latter washing zone of the last washing stage and fed to the latter washing zone of the second to last washing stage and used as wash liquid.
In all of the above-described apparatus at least the feed of the wash liquid, or the treatment of the filtrates, or both, have drawbacks, which may lead to poor washing. If a washer is not able to provide adequate washing, then a washer with more washing stages, or even a washer of a different type, is required. It may also be necessary to try to solve the problem by increasing the consumption of clean wash liquid so that there will be greater demand for steam in the evaporation plant and the capacity of the waste water treatment equipment, and also partly the environmental load, have to be increased.
The above-mentioned problems have been significantly dealt with in the manner described in FI patent application 954259 and WO patent application PCT/FI96/00316. In other words, in such a way that at least part of the filtrate obtained after the actual suction, press and/or thickening stage is directed to the preceding wash/washing stage to serve as wash liquid. Further, it is possible to solve these problems in such a way that in a multi-stage fractionating wash at least part of the filtrate from the suction, press, and/or thickening stage taking place after the actual wash is directed to the first zone of the preceding wash/washing stage to serve as wash liquid.
Further, in some mills the evaporation plant is not able to treat washing filtrate flows as large as would be produced by the washing methods according to the above-described patents and patent applications, i.e. in order to wash the pulp as clean as desired. In other words, when utilizing these washing methods, the brown stock would have to be washed at a lower dilution factor (i.e. with a smaller amount of wash liquid into the washer) than normally. Conventionally, the dilution factor in brown stock washing has been on the level of 2.0-3.5 m.sup.3 /ton of chemical cellulose pulp produced (ADT, or "air dried ton"). The dilution factor is calculated as the difference between the amount of fresh wash liquid brought to the washer per ton of chemical pulp to be washed and the amount of liquid discharged from the washer with the washed pulp. The unit of the dilution factor (DF) is m.sup.3 /ADT.
When it is necessary to lower the dilution factor, for example because of the capacity of the evaporation plant, the result from washing begins to deteriorate if the above-described washing methods are used. Should the end result from washing achieved under these conditions not be adequate, new washing capacity and/or evaporation capacity has to be added so as to sufficiently increase the dilution factor.
The invention seeks to solve the problems described above in a manner that is applicable to many different types of washers, and in such a way that it is possible to further intensify a cellulose pulp wash without having to invest in or construct more washing equipment.
According to the present invention a method of displacement washing cellulose (e.g. chemical, such as kraft, or mechanical, etc.) pulp in a washing system having a plurality of stages with a filtrate from each stage, the stages in sequence being N, N+1, through N+Z where Z is zero or a whole number greater than one, and the stages including filtrates F.sub.N, F.sub.N+1, through F.sub.N+Z, where Z is a whole number greater than one, is provided. The method comprises the steps of: (a) Feeding cellulose pulp to the washing system stages, in sequence, including the stages N and N+1. (b) Washing the cellulose pulp in the washing system, including stages N and N+1, and discharging the washed pulp from the washing system. (c) Feeding a wash liquid to each stage of the washing system. And (d) removing at least one filtrate from each stage of the washing system, including the filtrates F.sub.N and F.sub.N+1 ; and wherein steps (c) and (d) are practiced so that a part F.sub.N;x of the filtrate F.sub.N is combined together with all or part of the filtrate F.sub.N+1 to produce a combined filtrate, and the combined filtrate is fed to washing stage N during the practice of step (c). In the preferred embodiment the part F.sub.N;x of the filtrate F.sub.N is taken from the end of the washing stage N.
Further, stage N+1 typically includes at least two filtrates, F.sub.N;x+1I and F.sub.N;x+1II, and wherein the part F.sub.N;x of the filtrate F.sub.N from washing stage N is typically combined with the first filtrate F.sub.N+1;I from the next washing stage N+1 and is returned to stage N in the practice of step (c). Steps (c) and (d) are practiced for each of a plurality of washing stages. Also step (c) may be practiced using at least first and second wash liquids for stage N; and at least two different filtrates F.sub.N;x+1,I and F.sub.N;x+1,II may be taken from the end of stage F.sub.N+1 ; and at least two different filtrates F.sub.N;xI and F.sub.N;xII may be taken from the end of washing stage N and are combined with the first two filtrates F.sub.N+1;I and F.sub.N+1;II from the next washing stage N+1 to form a second combined filtrate; and the second combined filtrate may be returned to stage N to serve as the first and second wash liquids in the practice of step (c). In the preferred embodiment the part F.sub.N;x of the filtrate F.sub.N is between about 5-30% of the amount of the filtrate F.sub.N.
The invention may also be practiced so that at least two different filtrate parts F.sub.N;xI, F.sub.N;xII, are taken from the end of washing stage N, and are combined with at least two different filtrates F.sub.N+1;I, F.sub.N+1;II, from the beginning of the next washing stage N+1 to form a second combined filtrate, and the second combined filtrate is fed to washing stage N during the practice of step (c). Each of the parts F.sub.N;xI, F.sub.N;xII, is between about 5-30% of the volume of the filtrate F.sub.N, and typically the two different filtrates F.sub.N+1;I, F.sub.N+1;II, are collectively over 50% of the volume of F.sub.N+1.
It is the primary object of the present invention to provide an effective method of displacement washing which is versatile, can be used with a variety of different types of washing equipment, and is extremely effective. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.